The present invention relates to a phased array ultrasonic testing apparatus and testing method therefor for testing for internal flaws in an object made of a metal or nonmetallic material using an ultrasonic wave.
Conventionally, an ultrasonic testing apparatus has been proposed to test for flaws within an object. The apparatus of this type has also been applied to an ultrasonic diagnostic apparatus for examining a patient. The ultrasonic testing apparatus has an array probe comprising a plurality of ultrasonic transmission/reception transducers, and an electronic circuit for controlling the ultrasonic transmission/reception timing of these transducers. The testing apparatus of this type generally employs an electronic scanning system. The electronic scanning system includes linear scanning and sector scanning. Linear scanning is performed such that some of the plurality of transducers of ultrasonic probe are sequentially activated or excited. As shown in FIG. 1, ultrasonic beams 12 then propagate linearly within tested body 14. Flaw portion 16 inside tested body 14 is B-scope displayed as flaw image 20 on CRT display screen 18 (FIG. 2). On the other hand, according to sector scanning, ultrasonic transmission/reception timing of the transducers of ultrasonic probe 10 change sequentially. As shown in FIG. 3, ultrasonic beams 12 are scanned in a sector shape inside tested body 14. This beam scanning allows B-scope display of flaw portion 16 inside tested body 14 as flaw image 20 on CRT display screen 18 (FIG. 4).
In an ultrasonic testing, a tip echo technique is proposed as a technique for evaluating dimensions of a flaw on a surface of and/or inside of tested body 14. According to this technique, a conventional type ultrasonic probe 22 is used in place of an array probe. Now assume that ultrasonic beam 12 from probe 22 is scattered or diffracted at one tip 16A of flaw portion 16, as shown in FIG. 5. In this case, when a wave received by probe 22 is A-scope displayed on CRT display screen 18, diffraction echo component 26A appears in displayed waveform 24 (FIG. 6). Then assume that probe 22 is shifted to the right by predetermined distance X in FIG. 5. Diffraction echo component 26B caused by the other tip 16B of flaw portion 16 is A-scope displayed on CRT display screen 18 (FIG. 6). According to the tip echo technique, a predetermined operation is performed to evaluate the dimensions of flaw portion 16, using shift distance X of probe 22 in FIG. 5 and propagation path lengths of ultrasonic beams corresponding to echo components 26A and 26B.
An ALOK system is proposed as another technique for evaluation of the above flaw dimensions. In this system, a correlation between a shift distance of the probe and propagation path lengths of flaw echoes is computed as precisely as possible. The flow dimensions are evaluated from an obtained correlation. The following reference is available for the ALOK system: B. Grohs et al., "Characterization of Flaw Location, Shape and Dimensions with ALOK system", Material Evaluation, vol. 40-1 (Jan. 1982).
Two types of sector scanning methods are available: one wherein a test is made while ultrasonic beams are converged; the other wherein a test is made without converging ultrasonic beams. FIG. 7 shows a case of converged beam sector scanning. Referring to FIG. 7, main beam direction 30 of beams 12 converged by electronic control at single point F geographically differs from main beam direction 32 of beams collimated thereby. Even if a test is made under the condition that a transmission/reception beam index point of the ultrasonic beams is preset at array center C of probe 10, an actual transmission/reception beam index point becomes point A. As a result, a position of the B-scope display image indicating an internal flaw of tested body 14 is misaligned from the actual position. Referring to FIG. 8, flaw portions 16.sub.1 and 16.sub.2 exist at angular positions .theta.1 and .theta.2 with respect to the normal line through the array center or preset main beam transmission/reception beam index point C. Angular positions .theta.1.degree. and .theta.2.degree. of flaw images 20.sub.1 and 20.sub.2 displayed on B-scope CRT display screen 18 differ from the actual angular positions .theta.1 and .theta.2 due to this misalignment, as shown in FIG. 9. For this reason, according to the converged beam testing technique having array center C as the beam transmission/reception beam index point, internal flaw portions 16 in tested body 14 cannot be measured with high precision.
Furthermore, according to the tip echo technique described above, the position of the flaw portion inside the tested body cannot be detected. For this reason, when the tip echo technique is employed, the position of the internal flaw portion must be checked by a cut-and-try procedure. This cut-and-try procedure must be performed while probe 22 is precisely moved to ensure the ultrasonic beam is properly incident on flaw tips 16A and 16B (FIG. 5). Therefore, complicated probe scanning jigs must be used. However, even if a precise cut-and-try procedure is performed using such jigs, a large diffraction echo may not be obtained at a given angle of incidence of an ultrasonic beam on flaw tip 16A or 16B. There is no guarantee of alignment of the diffraction echo propagation direction with a high reception sensitivity direction of probe 22. In order to eliminate the above drawbacks, a plurality of ultrasonic probes having different transmission/reception angles must be prepared. The cut-and-try procedure must be performed in accordance with various combinations of these probes. Otherwise, a highly precise diffraction echo cannot be obtained and flaw dimensions cannot be evaluated.